It is frequently necessary to alert someone to a changing condition and provide graphical information for interpretation. For example, a hospital often needs to alert a doctor to a patient's changing condition and provide the doctor with the patient's electrocardiogram for diagnosis. Or, more generally, in a process control situation, such as a power generation plant or a manufacturing line, it can be beneficial to update an expert on the current conditions and the recent history. Many times, the expert may not be present on the site and thus must be contacted by a method not dependent on his location.
Facsimile transmission is one method of sending graphical information. The availability of cellular telephones, portable facsimiles, and batteries permit facsimile reception in the field. However, scanning for facsimile transmission can introduce noise and errors into the information. Even when a computer sends information using a "fax modem," thereby eliminating the printing and scanning steps, the facsimile process necessarily alters the scale and limits the resolution of the transmitted information. Furthermore, the combination of a facsimile receiver, cellular telephone, and the required batteries to power them would be so great that it would not be carried at all times.
Existing paging networks, paging receivers, and palmtop computers permit alphanumeric information to be conveniently received over broad areas. The small size of a Hewlett-Packard 95LX palmtop computer and an associated paging receiver allow the combination to be carried virtually everywhere.
Paging networks were not designed to transmit large block of binary data. Thus, they typically transmit only a limited 7-bit character set, thereby prohibiting the transmission of an unmodified graphical binary data file. Furthermore, a paging network may strictly limit message size to less than that of the typical graphical file.